Semiconductor-based image sensors, such as complementary metal oxide semiconductor (CMOS) image sensors, are commonly used as transducers for transforming visually viewable images into corresponding electrical signals. A semiconductor-based image sensor in general comprises light sensing elements and signal-processing electronics for processing electrical signals generated from the light sensing elements upon capturing an image, where the light sensing elements and the signal-processing electronics are built and integrated in a semiconductor substrate. In a typical realization, the light sensing elements are photodiodes and the signal-processing electronics are realized by CMOS transistors. Many variants of this typical realization are described by Gamal, A. E. and Eltoukhy, H. (2005), “CMOS image sensors,” IEEE Circuits & Device Magazine, pp. 6-20, May/June 2005, by Bigas, M., Cabruja, E., Forest, J. and Salvi, J. (2006), “Review of CMOS image sensors,” Microelectronics Journal, pp. 433-451, 2006, and by Ohta, J. (2007), Smart CMOS Image Sensors and Applications, CRC Press, 2007, the disclosures of all of which are incorporated herein by reference in their entirety.
The semiconductor substrate has a front side and a backside. The front side is typically grown with a high-quality epitaxial layer on which electronic components such as transistors and diodes are fabricated. It follows that typically the front side is the side on which the light sensing elements and the signal-processing electronics are fabricated. On top of the light sensing elements and the signal-processing electronics, typically there are one or more metallization layers or metal wires inter-separated by insulating dielectric layers.
In one option, an image is received on the front side of the semiconductor substrate of the image sensor, leading to a frontside-illuminated (FSI) image sensor. In this image sensor, the image is required to travel through the metallization layers so that some light energy is reflected back by metal wires therein, thereby reducing the chance of photons being captured by the light sensing elements and hence reducing the sensor sensitivity. This disadvantage can be avoided by using a backside-illuminated (BSI) image sensor. In the BSI image sensor, the backside of the semiconductor substrate is specially treated to remove a substantial amount of semiconductor materials on the backside such that the light sensing elements are located substantially close to the backside surface after treatment. When the light of an image reaches the backside, it only needs to penetrate a small amount of semiconductor materials before reaching the light sensing elements, thereby reducing attenuation of the image. In the art, removal of the substantial amount of semiconductor materials for a BSI image sensor with a conventional structure can be achieved by a mechanical grinding process or a chemical-mechanical planarization (CMP) process. Both of these processes involve precision machine tools to perform precision mechanical steps, and hence is in general costly.
There is a need in the art to have a new BSI image sensor without the need to involve a mechanical grinding process or a CMP process in fabrication.